See how your savings and investments grow over time. Add regular monthly contributions and watch the power of compounding unfold year by year.
📈 Compound Interest Calculator
Works for savings accounts, ISAs, investments, and pensions.
£
£
%
yrs
Final Balance
—
—
—
Initial Deposit
—
Total Contributions
—
Total Interest
—
Total Invested
—
Rule of 72
—
Return on Investment
—
📊 Growth Over Time
Principal
Contributions
Interest
📅 Year-by-Year Growth
Year
Balance
Added
Interest
Total Interest
What Is Compound Interest?
Compound interest is the process where the interest you earn is added back to your principal — and then you earn interest on that new, larger balance. Every period, your returns generate more returns. Over long periods, this creates exponential growth.
Contrast with simple interest: £10,000 at 7% simple interest for 30 years earns £21,000 (£700/year × 30). With compound interest, the same £10,000 at 7% grows to £76,123 — earning £66,123 in interest. Your interest earns interest, year after year.
The Rule of 72: Divide 72 by your annual interest rate to find how long your money takes to double. At 7%, money doubles every 10.3 years. At 4%, every 18 years. At 10%, every 7.2 years.
The Impact of Regular Contributions
Adding regular monthly contributions dramatically accelerates compound growth. Starting at 25 vs 35 with £200/month at 7% — the 25-year-old invests £24,000 more but ends up with roughly twice the final balance by age 65, purely from 10 extra years of compounding. Start early, even with small amounts.
Frequently Asked Questions
For UK cash savings accounts, current rates are 3–5% (2025). For stocks and shares ISAs investing in global equities, a commonly used long-term historical average is 6–10%. For a globally diversified index fund portfolio, 6–8% is a reasonable planning assumption — but this is not guaranteed and past performance does not predict future results. For inflation-adjusted real returns, subtract 2–3% from nominal rates.
Compound interest works against you on debt — your unpaid balance grows exponentially. Credit cards at 20–25% APR are particularly dangerous: a £3,000 balance at 22% with minimum payments takes over 15 years to clear and costs over £5,000 in interest. High-interest debt should always be prioritised before investing — no investment reliably returns 20%+ to compete with credit card rates.
No — this calculator shows gross (before-tax) growth. In the UK, basic rate taxpayers have a £1,000 Personal Savings Allowance (PSA). Interest above the PSA is taxed at your marginal rate. To model after-tax growth, reduce the interest rate by your effective tax rate on savings. Alternatively, invest within a Stocks and Shares ISA or Cash ISA — all returns inside an ISA are completely tax-free, making ISAs ideal for maximising compound growth.
Important: Calculator results are educational estimates only. Investment returns are not guaranteed and past performance does not predict future results. Always consult an FCA-regulated financial adviser before making investment decisions.